Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter, nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
Oxidation catalysts comprising precious metals such as iron-platinum group metals (PGM) dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a diesel powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and the soluble organic (SOF) fraction of particulate matter, oxidation catalysts that contain iron-platinum group metals (which are typically dispersed on a refractory metal oxide support) promote the oxidation of nitric oxide (NO) to NO2.
For example U.S. Pat. No. 5,491,120 discloses oxidation catalysts containing ceria and a bulk second metal oxide which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina.
U.S. Pat. No. 5,627,124 discloses oxidation catalysts containing ceria and alumina. It is disclosed that each have a surface area of at least about 10 m2/g. The weight ratio of ceria to alumina is disclosed to be 1.5:1 to 1:1.5. It is further disclosed to optionally include platinum. The alumina is disclosed to preferably be activated alumina. U.S. Pat. No. 5,491,120 discloses oxidation catalysts containing ceria and a bulk second metal oxide, which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina.
The prior art shows an awareness of the use of zeolites, including metal-doped zeolites, to treat diesel exhaust. US 2008/045405 discloses a diesel oxidation catalyst for the treatment of exhaust gas emissions, such as the oxidation of unburned hydrocarbons, and carbon monoxide and the reduction of nitrogen oxides. More particularly, US 2008/045405 is directed to a washcoat composition comprising two distinct washcoat layers containing two distinctly different weight ratios of Pt:Pd.
PGMs are the catalytic active species in DOC- and Three-Way Conversion—(TWC) Applications. Usually platinum and palladium are used as active metals in diesel oxidation catalysts and a combination of platinum, palladium and rhodium as active metals in TWC-catalysts. The oxidation of CO to CO2 and the oxidation of hydrocarbons to CO2 is mainly catalyzed by platinum. The addition of palladium decreases the mobility of the platinum on the surface of the refractory metal oxide support, for example γ-Al2O3. In addition, in the presence of palladium the sintering of the metals particles at higher temperatures can be reduced. In the TWC application rhodium is used in addition for catalyzing the reduction of nitrogen oxide (2 NO+2 CO→N2+2 CO2).
The accessibility of the surface of the catalytic active species is essential for the catalytic activity. The process for the preparation of diesel oxidation catalysts known in the art comprises (i) combining platinum and palladium salts in water (ii) adding γ-Al2O3 and further additives to the solution of (i) obtaining a water-based slurry (so called washcoat). The PGM-containing washcoat is adjusted to a special viscosity. Honeycomb cordierites are coated via dip coating with a PGM-containing washcoat and calcined. The reduction of the PGM-ions takes place while driveway. The described process leads to very small metallic PGM-nanoparticles having an average diameter of 1 to 2 nm. As the noble metal solution migrates deep into the pores of the alumina, even into the innermost layer of the alumina, the later on reduced PGM nanoparticles are also deposited there. Thus, the obtained PGM containing alumina shows a uniform distribution of the PGMs. However, the PGM being in the innermost layer of the alumina are not accessible for the catalyses.